1. Field of the Invention
The present invention is directed to a method and apparatus for measuring the mineral content of an ore slurry. More particularly, the present invention is directed to an on-stream method and apparatus for measuring, on a dry-weight basis, the mineral content of an aqueous ore slurry.
2. Description of the Prior Art
The conveyance of mined ores, by pumping an aqueous ore slurry through a pipeline, is widely practiced. For instance, when phosphate ore is mined from underground or surface mines, the latter initially involving the use of large drag line excavators which remove the overburden and then recover the crude phosphate ore which is known as "matrix", the matrix then being washed by hydraulic jets in an improvised sump pit, the resulting mixture of phosphate ore, water, sand and gravel, called the "slurry", is pumped by pipeline to a washing plant. In the washer, the slurry is pumped over a series of screens interspersed with log washers which act to break up clay balls and other large pieces in the matrix. Usually, there are three streams exiting the washer. All of these streams are also in the form of aqueous ore slurries. One is phosphate pebble product stream typically having a BPL of about 65% (The phosphate content of phosphate ore can be expressed in either of two ways. One is the percentage of bone phosphate of lime or "BPL", which is tricalcium phosphate, Ca.sub.3 (PO.sub.4).sub.2. The second manner of expressing phosphate content is in terms of phosphorus pentoxide, P.sub.2 O.sub.5. The ratio of BPL content to P.sub.2 O.sub.5 content is about 2.18 to 1.0) and a particle size within the range of about 1 millimeter to about 3/4 inch (+16 mesh). A second stream containing both phosphate values and insoluble siliceous minerals or gangue (i.e., sand) has an intermediate particle size range between about 0.1 and 1 millimeter (-16 mesh to +150 mesh). The third stream comprises clay slimes having a particle size below about 0.1 millimeter (-150 mesh). The slimes are typically discarded into a slime pond where the clay eventually settles. Of these three product streams, only the second is subjected to further processing.
In a conventional phosphate ore processing operation, the second stream is fed to a sizing apparatus which typically divides the phosphate and siliceous mineral containing fraction into three distinct particle size ranges. The finest of these three streams has a particle size range of about 0.1-0.4 millimeters (-35 mesh to +150 mesh). This stream is conditioned and then subjected to froth flotation benefication. Froth floation is beneficiation process whereby a particulate ore is slurried in an aqueous medium to form a pulp through which a gas, such as air, is sparged. One or more components of the ore is thereby selectively caused to rise to the surface of the slurry in a frothing chamber while the chamber is sparged with the gas. The particles are caught in the froth formed on the surface of the slurry in the chamber and are removed with the froth while particles that do not rise remain in the slurry and are drawn off through the bottom of the flotation chamber. In conventional froth flotation equipment, air or other gaseous medium is sparged through a tank containing an aqueous pulp of comminuted ore, frothing agents, collecting reagents and other frothing aids. The fine stream is subjected to froth flotation using for example well-known anionic fatty acid collecting reagents wherein fine rough siliceous tailings are removed (and wasted) and a fine rough phosphate concentrate is collected in the froth.
The intermediate particle size stream coming from the sizing apparatus has a particle size range of about 0.4-0.7 millimeters (-24 mesh to +35 mesh) and is fed to a coarse flotation unit which also uses conventional anionic fatty acid collecting reagents. In the coarse rough flotation, a coarse rough tailing is removed and wasted, and a coarse rough phosphate concentrate is collected in the froth and can be combined with the fine rough concentrate mentioned earlier.
The combined streams of the fine rough concentrate and the coarse rough concentrate then generally are sent to an acid (typically H.sub.2 SO.sub.4) scrubbing unit to remove the fatty acid and other reagents. The acid scrubbed slurry then is washed with water and sent to a cationic (amine) flotation unit. Before the cationic flotation, the feed slurry is conditioned with a flotation reagent typically comprising a mixture of an amine and kerosene.
The third stream exiting from the sizing apparatus comprises particles having a very coarse particle size in the range of about 0.7-1.0 millimeters (-16 mesh to +24 mesh). The stream is usually beneficiated by a combination of chemical conditioning and mechanical separation techniques using a skin flotation device such as a spiral separator, a belt separator, a concentrating table or the like. Normally, this stream is chemically conditioned at a high solids concentration with a conventional anionic conditioning reagent such as a mixture of a fatty acid reagent, such as tall oil, and a fuel oil extender. The conditioning reagent may also include ammonia or caustic for pH control. The stream then is fed to the skin flotation device which comprises a series of downwardly sloping spiral troughs having a number of side exits ports in the trough along the inner edge thereof. The heavier siliceous materials contained in the slurry tend toward the inside of the spiral trough while the lighter reagentized phosphate materials (with attached air bubbles) tend toward the outside. The inside exit ports are positioned to accomplish separation of the heavier siliceous materials. The spiral tails containing the siliceous minerals are then sent to a scavenger flotation cell wherein residual phosphate values are foamed to the top, while the heavier siliceous minerals are wasted from the bottom of the cell.
The spiral concentrate streams and the scavenger flotation cell streams generally then are combined to produce another stream typically having a BPL value of about 68% and an insoluble fraction of about 8-12%.
In the beneficiation of mineral slurries, such as phosphate ore slurries, it is extremely advantageous to know the precise mineral content of the solids contained in the slurry. For example, in the beneficiation of phosphate containing ore, it is desirable to know the phosphate content of the ore on a dry weight basis. This information is especially desirable in monitoring the froth flotation beneficiation of ore slurries, such as in the fine flotation, the coarse flotation and the scavenger flotation of phosphate ores mentioned above. The reason for this is that collecting reagents represent a significant part of the cost of ore beneficiation by froth flotation and the optimum amount of collecting reagent added to the slurry is dependent upon the amount of phosphate, on a dry weight basis, contained in the slurried ore. The amount of collecting reagent added is generally a function of the BPL content of the solids of one or more of the flotation feed and exit streams. In order to optimize the amount of collecting reagent added to a froth flotation cell in the beneficiation of phosphate ore, the percent BPL or percent insolubles contained in either the flotation concentrate or tails streams should preferably be determined as precisely as possible.
To date, two methods for controlling froth flotation ore beneficiation processes have been used. First, samples of the flotation exit streams (both the flotation concentrate stream and the flotation tail stream) are periodically taken and sent to an on-site laboratory to determine the weight ratio of phosphate to sand in the solids portion of the slurry. This type of chemical analysis typically requires a minimum of about two hours, during which the sample is first dewatered and then completely dried in an oven and then digested using hydrochloric and nitric acids at high temperatures in order to determine the phosphate content of the solids contained in the slurry sample. The major portion of this analysis time is spent in dewatering the slurry sample and drying the solids. Because the analysis takes a minimum of about two hours, the slurry samples are "composite samples" comprising an average of the flotation exit stream over the sampling period with small samples being taken about every few minutes. In the froth flotation beneficiation of phosphate ores, the laboratory analysis gives an average value of the phosphate content of the solids contained in the slurry over the sampling period.
Secondly, the flotation operator takes instantaneous samples from the flotation feed and exit streams and makes a rough visual estimate of the sand and/or phosphate content of the sample. In the beneficiation of phosphate ores, phosphate rock has a highly variable color and appearance, ranging from almost pure white to a dark grayish color. In other cases, the phosphate rock can have an appearance quite close to that of the sand. Accordingly, control of the flotation process based upon the operator's visual inspection of the flotation feed, concentrate and tail samples inherently involves a great deal of guess work. Under the best of conditions, an experienced phosphate flotation cell operator can only "see" differences in the percent BPL of a flotation tail slurry (and differences in the percent acid-insoluble solids of a flotation concentrate slurry) on the order of about 2% or more.
It is accordingly an object of the present invention to provide an on-stream method and apparatus for quickly and accurately measuring the content of a mineral contained in a slurried ore, on a dry weight basis.
It is another object of the invention to provide a method and apparatus for quickly and accurately determining the mineral content of a slurried ore, on a dry weight basis, in order to improve mineral recovery and to provide more effective control of the processing of the ore, such as assuring optimum reagent use in a froth flotation process.
It is a more specific object of the present invention to provide an on-stream method and apparatus permitting automated control of a froth flotation process, particularly in the froth flotation of phosphate ores, which does not rely on the subjective evaluation by an operator of froth flotation feed and/or exit streams nor on the time-consuming dewatering, drying and chemical analysis used in the past.